Circuit arrangement for acquisition of signals from an apparatus for measuring beams of charged particles for external radiotherapy

ABSTRACT

A circuit arrangement for acquisition of signals from an apparatus for measuring beams of charged particles for external radiotherapy, in particular protons, carbon ions, and other ion species, emitted by particle accelerators, comprising at least one ionization-chamber sensor ( 10 ), which includes a plurality of sensor channels ( 20 ), said circuit arrangement ( 200; 500 ) comprising a plurality of channel branches (B 0 , . . . , B 63 ; BR 0 , . . . , BR 63 ) in parallel designed to be connected to said sensor channels ( 20 ) for receiving respective measurement signals (i) therefrom, said channel branches (B 0 , . . . , B 63 ; BR 0 , . . . , BR 63 ) comprising respective current-to-frequency converters ( 210 ) and counters ( 220 ) for supplying count values (CT), representing a charge associated to a given channel ( 20 ), to a multiplexer ( 250; 550 ). According to the invention, said channel branches (BR 0 , . . . , BR 63 ) supply their own outputs (CT 0 , . . . , CT 63 ) directly to said multiplexer ( 550 ) and to an adder structure ( 240 ) comprising at least one first column ( 240   1 ) of adders ( 241   1 ) that adds up the values at the outputs (CT 0 , . . . , CT 63 ) of the channel branches (Br 0 , . . . , BR 63 ) to supply sum values (CT_ 100 , . . . , CT_ 115 , CT_ 200 , . . . , CT_ 203 , CT_ 300 ) at outputs thereof, said multiplexer ( 550 ) being configured for selecting, from the direct outputs (CT 0 , . . . , CT 63 ) of the channel branches (BR 0 , . . . , BR 63 ) and the outputs (CT_ 100 , . . . , CT_ 115 , CT_ 200 , . . . , CT_ 203 , CT_ 300 ) of the adder structure ( 240 ), a signal to be supplied to a readout electronics ( 400 ) for reading the channels ( 20 ), in particular in order to enable the channels necessary for detecting a given peak excursion via the multilayer-ionization-chamber sensor ( 10 ) without saturating.

TECHNICAL FIELD

The present description relates to a circuit arrangement for acquisitionof signals from an apparatus for measuring beams of charged particlesfor external radiotherapy, in particular protons, carbon ions, and otherion species, emitted by particle accelerators, comprising at least oneionization-chamber sensor, which includes a plurality of sensorchannels, said circuit arrangement comprising a plurality of channelbranches in parallel designed to be connected to said sensor channelsfor receiving input signals, said channel branches comprising respectivecurrent-to-frequency converters and counters for supplying count values,representing a charge associated to a given channel, in particular to asingle parallel output, via a multiplexer.

TECHNOLOGICAL BACKGROUND

In the field of external radiotherapy with charged particles,adrotherapy, which uses protons and carbon ions and other ion species,is one of the most advanced therapies, affording a finite penetrationdepth, low deposition of energy at input and marked fall-off, namely,the distal decay of the distribution of the dose. However, some of itsbenefits may become a risk for the patient on account of theuncertainties during administration of the treatment. It is hence offundamental importance to carry out monitoring during theradiotherapeutic treatment and regular calibration of the physicalparameters of the beam of particles.

This is typically carried out by installing at output from the particleaccelerator ionization chambers (with a low value of water equivalence,typically less than 1 mm) and via Quality Assurance (QA) procedures,which are carried out on a daily basis in health-care facilitiesequipped with different devices and instruments for each parameter to beverified.

The interaction of proton and ion beams with human tissue (which ismainly made up of water) enables the majority of the dose to be conveyedto a precise depth, following the profile of the so-called Bragg peak.In this way, it is possible to increase the precision on the target,limiting the dose that may reach healthy tissues. Moreover, the distaland lateral fall-off of a proton beam is considerably better than thelateral penumbra caused by a photon beam, enabling a fast decay of thedose in the vicinity of adjacent critical structures. As a consequence,the total energy deposited in a patient for a given target dose is lowerthan that of conventional treatments using photons. However, the Braggpeak for a single-energy proton and ion beam is so narrow that only alimited interval of depth can be treated with a very high dose. In orderto widen the interval of treatment depth and supply a uniform dose onthe tumour, a spread-out Bragg peak (SOBP) is created as set of purepeaks sent at a decreasing depth (by varying the energy of theparticles) and with a reduced dose to obtain the desired modulation.FIG. 11 shows the dose DO from non-modulated proton beams (pure Braggpeak BP) and modulated proton beams (spread-out Bragg peak SOBP). Italso shows the sets of peaks SP, the amplitude of which is weighted.FIG. 11 also indicates the interval EI of deposition of the energy ofthe spread-out peak.

Since protons and ions deposit their energy dose in a relatively smallvolume, corresponding to the interval EI, it is of fundamentalimportance to verify correctly the position of the interval EI ofdeposition of the beam of particles prior to treatment of patients.Calibration of the instrument (size, shape, and intensity of the beam)and verification of the depth-dose curves are carried out during the QAprocedures.

According to the prior art, it is known to use for this purpose smallionization chambers or diodes that move through dummies made oftissue-equivalent materials (for example, water or perspex). In thiscontext, it has been suggested to use multilayer ionization chambers(MLICs) in order to accelerate the QA procedures in health-carefacilities, as is described, for example, in Lin, S. et al., (2009) “Amultilayer ionization chamber for proton beam Bragg peak curvemeasurements”, Proceedings of the International Conference of theParticle Therapy Co-Operative Group (PTCOG), Heidelberg.

The principle that superintends use of the above apparatuses is thepossibility of measuring the charge deposited on each of the anodes (orcathodes) of the various layers of the device while the beam ofparticles passes through a stack of calibrated water-equivalentabsorbers. This enables instantaneous evaluation of the depth-dosedistribution of the beam (whether single-energy beam or spread-out Braggpeaks) by virtue of the simultaneous reading of all the ionizationchambers that make up the MLIC device. The typical structure of an MLICis illustrated in FIG. 1.

FIG. 1 shows in this connection an MLIC sensor 10, which comprises aplurality of sensor structures, or channels, 20, in FIG. 1 20 ₁, . . . ,20 _(N), comprising pairs 15 ₁, . . . , 15 _(N) of anodes 11 andcathodes 12, separated by an ionization chamber 14, i.e., a space thatidentifies the ionization region, and followed by an absorber layer 13.

The total number of channels identifies the maximum energy range of theparticles that can be measured, whereas the materials and the physicalthicknesses of each channel determine the water-equivalent thickness ofthe MLIC sensor 10. Known MLIC sensors comprise a fixed number ofchannels, for example 128 or 180 channels.

FIG. 2 illustrates an accelerator 1000, which is able to generate a beamof charged particles 1300 conveyed through a tunnel 1100 and an outletmouth 1200 in the direction of a spatial region P, which in the figureis represented as being cylindrical of a length equal to the interval EIof deposition of the energy of the spread-out peak, even though ingeneral it may assume also other shapes. This spatial region P islocated, for example, above a therapy table 1400 on which a patient tobe treated can be positioned. In the spatial region P, for purposes ofcalibration, an apparatus for the calibration of beams of chargedparticles 100 is positioned in such a way that the beam of particles1300 passes through MLIC sensors 20 that make up the calibrationapparatus 100. This apparatus 100 is modular; i.e., it comprises aplurality of aligned sensor modules.

In FIG. 2 it may be noted how the modular MLIC calibration apparatus 100comprises an outer casing 110, of a substantially parallelepipedalshape, which comprises within it a horizontal stack of modular elements120, each including an MLIC sensor 10, or another type of sensor formeasuring the position and size of the beam of particles, and asupporting frame for the aforesaid sensor.

The calibration apparatus 100 enables instantaneous evaluation of thecharacteristics of the therapeutic beam of particles 1300 in thedirections X, Y, and Z, where Z corresponds to the direction along whichthe depth D is evaluated.

In this framework, where the instantaneous flow is very high and theefficiency of the calibration and measurement apparatuses must becontrolled and possibly corrected, and moreover, the readout front-endof the detector and of its channels must be able to cover the entirerange of expected input signals, particular attention must be dedicatedto the circuit for acquisition of the signal from the channels of thedetector or sensor.

Generally known are ASIC (Application-Specific Integrated Circuit)circuit solutions based upon a count-type charge converter.

These circuits comprise, in the first place, a plurality of channelcircuits. Each channel circuit converts the input charge into anincrement of a given number of counts of a purposely provided counter,where a fixed amount of charge, in what follows referred to as “quantumof charge”, corresponds to the count of one.

A schematic representation of a channel circuit 205 is shown in FIG. 4.

An input current i is integrated by means of an integrator circuit 201comprising an operational transconductance amplifier (OTA) 202, an inputresistance Rin connected to the inverting terminal of the OTA 202, and acapacitance Cint between the inverting terminal and the output of theOTA 202. An output voltage Vout of the integrator 201, at output fromthe OTA 202, increases when the input current i is negative, i.e., itcomes out of the channel circuit 205, whereas it decreases otherwise.The output voltage Vout of the integrator 201 is compared with two fixedthresholds, a high one VTH and a low one VTL, via two synchronouscomparators CMP_1 and CMP_2, the other input of which is connected tothe output voltage Vout of the integrator 201. The threshold voltage ofeach of the two comparators CMP_1 and CMP_2 is set from outside: thehigh threshold voltage VTH is the voltage threshold of the firstcomparator CMP_1 that operates on negative input currents i, whereas thelow threshold voltage VTL regards the comparator CMP_2, which is activefor positive input currents. The value of the thresholds VTH and VTLdoes not have a particular influence on operation of the channel circuit205 as long as the voltage variation remains within the output range ofthe OTA 202 and as long as the difference VTH-VTL between the twothresholds is greater than the voltage jump caused by subtraction of thecharge quantum.

The comparators CMP_1 and CMP_2 have their own outputs connected to twocontrol inputs of a pulse generator PG. Appearing in FIG. 4 is also aperiodic clock signal clk, which provides a synchronisation referenceboth to the comparators CMP_1 and CMP_2 and to the pulse generator PG.

The input node A of the OTA 202 is connected to a reference voltage VR,which is also the node on which the current pulse of polarity oppositeto the one used for subtraction of charge is discharged, operatingthrough two switches in series, a first switch sw1 and a second switchsw2.

The reference voltage VR is a voltage that is connected to guard ringsin the sensor to ensure that any possible charges that are not directlygenerated by ionization, but are for example generated by the differenceof potential present between layers, are collected by the guard rings,without being transmitted to the layer on which the measurement signal(i.e., the current i) is picked up, which thus collects prevalently justthe charges produced by ionization of the gas traversed by the beam ofcharged particles.

The channel circuit 205 further comprises a charge-control capacitanceCsub, one terminal of which is connected to the node identified betweenthe first switch sw and the second switch sw2.

Whenever the input voltage of the comparator CMP_1 or CMP_2, i.e., theoutput voltage Vout of the integrator 201, crosses the respectivethreshold VTH or VTL, the corresponding comparator CMP_1 or CMP_2 setsat its output a given logic level for the input of a pulse generator PG.As long as this input level is set, the pulse generator PG sends to acapacitor Csub a positive pulse PC on its other terminal, as shown inFIG. 5. The voltage amplitude of the pulse PC, ΔVpulse, is defined asthe difference between two reference voltages Vp+ and Vp−, which are setfrom outside.

The total capacitance Csub may be obtained with three capacitors inparallel, of 50, 100, and 200 fF respectively, which may be addedindependently, so that the charge-control capacitance Csub can beselected with each value between 50 and 350 fF in steps of 50 fF, via acapacitance-selection signal Cap_sel. The output response of thecapacitance Csub to a voltage pulse PC is represented by two currentsignals of opposite sign, δ+ and δ−, associated to which are charges Q+and Q−, respectively, according to the following relation (1):

Q+=Csub·ΔVpulse

Q−=Csub·(−ΔVpulse)  (1)

The timing of the first current signal δ+ is determined by the risingedge of the pulse PC, whereas the negative current signal δ− correspondsto the falling edge of the pulse.

By operating on the timing of the command pulses P1 and P2 of theswitches sw1 and sw2, the charge Q+ or Q− can be directed either to theinput of the OTA 202 or else to the reference voltage VR, adding orsubtracting a fixed amount of charge at each pulse P1, P2 generated bythe pulse generator PG. The decision on which of the charge signals isto be sent to the OTA 202 depends upon the output of the comparators, inorder to remove a fixed amount of charge from the capacitor Cint of theintegrator. This results in a change in the voltage drop on thecapacitor Cint, which is given by Q/Cint.

FIG. 5 shows a timing chart that represents the pulse PC sent to thecapacitor Csub, the command pulses P1, P2 of the switches sw1, sw2, andthe currents δ+ and δ− in the switches.

If, after the action described above, the input voltage, i.e., theoutput Vout of the integrator 201 of the comparator CMP_1 (or CMP_2)remains above (below) the threshold VTH (or VTL), the pulse generator PGcontinues to emit pulses PC and stops when the voltage Vout passes againbelow (above) the threshold.

In parallel, the pulse generator PG sends a count pulse, which may be asignal for increment Cnt_Up or a signal for decrement Cnt_Down of theup/down counter 220, according to which is the comparator that is actingat its input.

All the operations are synchronized via the external master clock clkand controlled via a digital finite-state machine (FSM) implemented inthe generator block PG and not shown in FIG. 4. If at a cycle of theclock signal the pulse generator PG detects that the comparator CMP_1(or CMP_2) is active, the next two clock cycles are used for generatingthe pulses for charge subtraction and command of the switches sw1/sw2.Two supplementary clock cycles are required before re-activation of theFSM in the pulse generator PG in order to leave time for the OTA 202 toreduce the output voltage. Thus, to generate a count, five master-clockpulses are required.

The relation between the frequency of counts ν and the input current iis:

$\begin{matrix}{v = \frac{i}{Q_{c}}} & (2)\end{matrix}$

where Q_(c) is the quantum of charge, which is given by:

Q _(c) =C _(sub) ·ΔVpulse  (3)

Reading of the total charge collected in the detector is provided by thenumber of counts generated during the measurement time multiplied by thevalue of the charge quantum Q.

FIG. 6 shows a known solution of circuit arrangement, designated by 200,which envisages operating, for example, with a measurement apparatus 100that has available a number M of channels 20 comprised in one or moreMLIC sensors 10 arranged in a horizontal stack, as described withreference to FIG. 2. It is here emphasized how reference to a sensor 10of an MLIC type is provided purely by way of non-limiting example, itbeing possible for the sensor to be also of some otherionization-chamber type suitable for detecting the intensity of beams ofcharged particles and configured for sending its own measurement signalthrough a plurality of measurement channels 20, for example a pixelionization-chamber detector. The circuit arrangement 200 comprises anumber N equal to 64 of inputs in0, . . . , in63, on each of which achannel 20 can be connected for conveying thereto a correspondingcurrent i collected by the channel 20 of the MLIC sensor 10.

Each of the above inputs in0, . . . , in63 in the circuit arrangement200 is the input of a respective circuit branch B0, . . . , B63. Eachk-th branch Bk comprises a channel conversion circuit 205 including arespective current-to-frequency converter 210, which converts therespective current i at input to the k-th branch into a frequency ν_(k).This frequency ν_(k) is then measured by a counter 220, comprised in thek-th branch Bk, which supplies at output the value of the counter CT, inparticular a 32-bit value.

The values assumed by the counters CT0, . . . , CT63 of the branches B0,. . . , B63 are supplied as inputs to a multiplexer 250, which, uponcommand from a 6-bit selection bus CS, enables selection of the channel20 from the channels connected to the inputs in0, . . . , in63, i.e., ofthe branch from the branches B0, . . . , B63, to be sent at its ownoutput as output data O and to be acquired via a readout electronics 400not shown in FIG. 4. It should be noted that, in effect, thecurrent-to-frequency converter implemented by the circuit arrangement200 corresponds to a charge-to-count converter.

As may be noted, in the circuit arrangement 200 of FIG. 4 also shown atinput are the signals, supplied by an external controller, that have thefunction of controlling the channel circuits 205, hence the thresholdsVTH, VTL of the comparators, the voltages VP+, VP−, thecapacitance-selection signal Cap_sel, in addition to the signal on theselection bus CS of the chip and the selection circuits of themultiplexer 250. The circuit 200 moreover receives a latch signal, L,which has the function of loading at a given instant the output of thecounters 220 into as many registers, and moreover an integrator-resetsignal RA and a counter-reset signal RD. The integration capacitor Cintof each channel circuit 205 may in fact be discharged through a commondigital reset input RA. Likewise, all the counters 220 may be zeroed viaa common reset asynchronous digital input, RD.

The circuit of FIG. 4 is described, for example, in the paper by La Rosaet al., Nuclear Instruments and Methods in Physics Research A 583 (2007)461-468 and manages, via a single channel branch, a respective channel.

However, in this type of circuit, even using a maximum conversion rateof 20 MHz and configuring the quantum of charge to the maximum value(for example, 1.155 pC), the maximum current that a branch can convertbefore saturating is less than 24 μA. This value is too low forapplications with pulsed accelerators, such as synchrocyclotrons. Withthese instruments, with a duty cycle of the pulse of a few units perthousand, the instantaneous current during the pulse must reach valuesgreater than those of current applications with linear accelerators,even by two or three orders of magnitude.

Consequently, the known solutions present limits in the values ofamplitude of current on the channels that they can manage.

OBJECT AND SUMMARY

The object of embodiments described herein is to improve the apparatusesand methods according to the known art as discussed previously.

Various embodiments achieve the above object thanks to a circuitarrangement having the characteristics recalled in the ensuing claims.

Various embodiments also refer to a corresponding measurement apparatusand method.

The claims form an integral part of the technical teachings providedherein in relation to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will now be described, purely by way of example,with reference to the annexed drawings, wherein:

FIGS. 1, 2, 3, 4, 5, and 6 have already been described previously;

FIG. 7 shows a principle circuit diagram of the circuit arrangementdescribed;

FIG. 8 shows a diagram representing the circuit arrangement describedand corresponding control signals.

DETAILED DESCRIPTION

In the ensuing description, numerous specific details are provided inorder to enable maximum understanding of the embodiments provided by wayof example. The embodiments may be implemented with or without specificdetails or else with other methods, components, materials, etc. In othercircumstances, well-known structures, materials, or operations are notillustrated or described in detail so that aspects of the embodimentswill not be obscured. Reference, in the course of the presentdescription, to “an embodiment” or “one embodiment” is meant to indicatethat a particular structure, feature, or characteristic described inconnection with the embodiment is comprised in at least one embodiment.Hence, phrases such as “in an embodiment”, “in one embodiment”, or thelike, that may appear in various points of this description do notnecessarily refer to one and the same embodiment. Moreover, theparticular structures, features, or characteristics may be combined inany convenient way in one or more embodiments.

The notation and references are provided herein only for convenience ofthe reader and do not define the sphere of protection or the scope ofthe embodiments.

In brief, the circuit arrangement proposed herein envisages providing,for a plurality of channels, a plurality of channel circuits comprisingcurrent-to-frequency converters and counters for supplying a respectivechannel count, stored in a respective channel register, as well as anadder-tree structure comprising at least one first column, or row, ofadders that adds up the outputs of sets of channel registers, inparticular comprising further columns of adders that add up the outputs,which are, in particular, stored in respective registers of the adders,of the adders of the previous column. The outputs of the channelregisters and of each adder of each column are supplied to amultiplexer, which can in this way enable the channels necessary fordetecting a given peak excursion without saturating.

FIG. 7 shows an embodiment 500 of a circuit arrangement for acquisitionof signals from an apparatus for calibration of beams of chargedparticles for external radiotherapy according to the invention.

As may be seen in FIG. 7, the architecture of the above circuitarrangement 500 comprises a plurality of branches BR0, . . . , BR63 forthe plurality of inputs in0, . . . , in63, it being possible for each tobe connected to a measurement channel 20 of an ionization-chamber sensor10 in order to receive a respective measurement signal, in particular acurrent signal. Each k-th branch BRK comprises a channel circuit 205,like the one shown in FIG. 4, hence comprising a voltage-to-frequencyconverter 210 and a counter 220, as well as a channel register 230. Alsoin this case the current-to-frequency converter implemented by thecircuit arrangement 500 corresponds, in actual fact, to acharge-to-count converter.

Unlike the circuit arrangement 200 of FIG. 6, the plurality of branchesBR0, . . . , BR63 are connected to a multiplexer 550 either directly,i.e., the N=64 outputs, which are the values of the counters CT0, . . ., CT63, of the channel registers 230 are connected to 64 inputs of themultiplexer 550, or through an adder structure 240. The adder structure240 receives at input the outputs, or count values, CT0, . . . , CT63,of the channel branches BR0, . . . , BR63. Each channel branch BRkreceives a measurement signal, in particular a current, at its inputink, determined by the connection to this input ink of the measurementchannel 20, and supplies an output, i.e., a corresponding count CT. Asdiscussed hereinafter, one measurement channel 20 can be connected to anumber of inputs of the circuit arrangement 500; hence, an input ink maynot receive the entire current of a channel 20. The adder structure 240supplies at output sums of the measurement signals present at the inputsin0, . . . , in63, or at a part of the inputs in0, . . . , in63. Inparticular, the adder structure 240 supplies at output the data comingfrom sums of counts corresponding to the sum of:

-   -   sets of four inputs, the sixteen signals CT_100, . . . , CT_115;    -   sets of sixteen inputs, the four signals CT_200, . . . , CT_203;    -   sum CT_300 of all the counts corresponding to all the 64 inputs        at input to the circuit arrangement 500.

In this way, it is possible to extend the current range at input to thecircuit arrangement 500 from a few microamps to hundreds of microamps,simply by selecting one of the addresses of the multiplexer 550 andconnecting in parallel the set of inputs the sum of which is containedin that address. In particular, as will be discussed in greater detailin what follows, one of the addresses is selected corresponding to a sumof a number of inputs ink for one and the same measurement channel 20that are such as to divide the current on each of the aforesaid inputsbelow a saturation value. The direct signals CT0, . . . , CT63, and sumsignals CT_100, . . . , CT_115, CT_200, . . . , CT_203, CT_300 at inputto the multiplexer 550 are shown in FIG. 8. The multiplexer 550 in thisway has 85 inputs: 64 inputs from the individual channels, 16 inputs assum of four channel branches, 4 inputs as sum of 16 channel branches(sum of four which are sum of four) and one input as sum of all thechannel branches (sum of four which are sum of sixteen), which is notshown in FIG. 7 and is in part visible in FIG. 8.

In greater detail, the adder structure 240 comprises an adder-treestructure including a first column 240 ₁, or row, of adders 241 ₁ thatadds up the outputs of sets of channel registers 230, specifically setsof four channel registers 230, and stores the result of the sum in arespective sum register 242 ₁.

The adder system 240 comprises further columns of adders 240 _(i),specifically, in the example, two further columns of adders 240 ₂ and240 ₃, which add up the outputs, in particular stored in the respectiveregisters of the adders 242 _(i), of the adders 241 _(i-1) of theprevious column 240 _(i-1). Hence, in the circuit arrangement 500, theoutputs of the sixteen registers 230 ₁ of the first column of the adders241 ₁ are supplied four by four to the four adders 241 ₂ of the secondcolumn 240 ₂. The outputs of the four registers 230 ₂ of the secondcolumn of the adders 241 ₂ are supplied to a single adder 241 ₃ of thethird column 240 ₃. It is clear how the number of columns of adders andthe number of adders in each column may be set even in a different way,according to the characteristics of the signals of the channels and ofthe measurement apparatus. The outputs of the first, second, and thirdcolumn of adders are supplied to the multiplexer 550, which can, in thisway, under the control of the selection signal 550, enable the channelbranches necessary for detecting a given peak excursion withoutsaturating.

As already anticipated, FIG. 8 is a partial representation. In fact, itshows only the structure that, starting from the first four signals CT0,. . . , CT3, generates the sum signal CT_100 at output from the adder242 ₁, as well as the path of the sum signal CT_100 in order to generatethe sum signals CT_200 and CT-300, through subsequent adders 242 ₁ and242 ₃. As regards the remaining direct signals CT0, . . . , CT63 and sumsignals CT_100, . . . , CT_115, CT_200, . . . , CT_203, CT_300, only thelast signals CT63, CT_115, CT_203 are indicated in a schematic principlerepresentation.

The circuit arrangement 500 is hence used as follows: in the case whereon a given channel 20 of a sensor 10 a current is expected such as tosaturate the single branch Bk, for example a current with a value thatis twice the saturation current that can be withstood by the singlebranch Bk, the channel 20 is connected in parallel to a plurality ofbranches; for example, the signal of a given channel 20 is connected tothe branches B0, . . . , B3 so that the current is divided on the fourbranches B0, . . . , B3, thus preventing saturation of the channelcircuits. In this case, the multiplexer 550 is configured for selecting,for the given channel 20, the output CT100 of the first column 240 ₁ ofadders 241 ₁. It is clear that, if, also other channels 20 are expectedto have a current that is twice the saturation current, these areconnected to other sets of branches that come under one and the sameadder 241. Likewise, if the current supplied is higher, such as tosaturate a channel circuit 205 even if the current is divided by four,it is possible connect the same given channel 20 to sixteen branchesgrouped in one and the same adder of the second column (selecting theoutput CT_200 in the multiplexer 550) or even, in the case of a muchhigher current, it is possible to connect the same given channel 20 toall sixty-four branches B0, . . . , B63, selecting the output that addsup all the branches, CT_300.

In other words, the circuit arrangement 200 comprises inputs in0, . . ., in63 pre-arranged for being connected in sets of two or more at outputfrom a measurement channel 20 that supplies a measurement signal i. Thisarrangement may take on, for example, the form of a simple electricalconnection between the output of the measurement channel 20 and a set ofthe inputs in0, . . . , ink, for example via an input terminal, orcommon node, connected at output from the measurement channel 20,departing from which are electrical cables or wires in parallelconnected, either directly or via passive elements such as electricalresistances, to the respective inputs of the set. The cables or wiresmay, in particular, be connected manually, in so far as in general themeasuring time does not entail the need to modify in an automated waythe connections of the signals of the measurement channels 20 to theinputs in0, . . . , ink of the circuit arrangement 200. It is, however,also possible to have, alternatively, a demultiplexer structure, whichreceives at input a measurement signal of one channel 20 and connectsit, under the control of a respective control signal, governed forexample by a computer, to a single input or else to two or more inputsin0, . . . , ink in parallel. This control signal of the demultiplexercan hence be co-ordinated with the selection signal of the multiplexer550 to connect simultaneously one measurement signal to a set of inputsand to select the output of the multiplexer corresponding to the sum ofthe set of branches BR0, . . . , BRK corresponding to the set of inputsin0, . . . , ink. Hence, it is possible to use a manual or automaticelectrical connection arrangement for connecting a sensor channel 20 toa set of inputs of the plurality of inputs in0, . . . , ink of thedevice 200.

Hence, more in general, an apparatus is provided for measurement ofbeams of charged particles for external radiotherapy, which comprises atleast one sensor channel or measurement channel 20, the output of whichis connected, in particular via a manual or automatic electricalconnection arrangement, in parallel to at least one set of inputs in theplurality in0, . . . , ink of the circuit arrangement 500 (or 200), andthe multiplexer 550 is configured for selecting the outputs CT_100, . .. , CT_115, or CT_200, . . . , CT_203 or CT_300 of the adder structure240 that adds up the values to the outputs CT0, . . . , CT63 of thechannel branches BR0, . . . , BR63 corresponding to the set of inputsin0, . . . , ink and for supplying them to the channel-readout circuit.

It should be noted in this context, where a number of channels areconnected together at the input of the OTA, the input resistance Rin onthe channel circuit 205 is particularly useful.

Hence, it is envisaged more in general to carry out, via the circuitarrangement 500, a measurement method that comprises connecting a givenchannel 20 of a sensor 10 to a respective branch or to a set of branchesBR0, . . . , BR63, in the example it being possible for the set tocomprise four, sixteen, or sixty-four adjacent branches, of the circuitarrangement 500 that converges into one or more columns, in particular240 ₁, 240 ₂, or 240 ₃ of adders 241 as a function of the value of amaximum current expected on that channel 20 of the sensor 10,configuring, via a 7-bit selection bus, the multiplexer 550 to select anoutput, corresponding to connection of that given channel 20, from amongthe direct outputs CT0, . . . , CT63 of the channel branches BR0, . . ., BR63 and the outputs CT_100, . . . , CT_115, CT_200, . . . , CT_203,CT_300 (i.e., the outputs of the respective sets of four, sixteen, andsixty-four adjacent branches) of the adder structure 240, as signal tobe supplied to a readout electronics 400 for reading of the channels 20.In particular, the operation of connecting a given channel 20 of asensor 10 to a respective branch or to a set of branches of the circuitarrangement 500 comprises selecting for the connection a set of brancheshaving a number of branches, in the example four, sixteen, orsixty-four, such as to divide on each branch a current lower than asaturation current of the channel circuit 205.

Note that the third column 240 ₃ is not shown in FIG. 7, whereas it isshown in FIG. 8.

Reading of the counters 230 can be carried out independently from anyother operation.

FIG. 8 shows the circuit arrangement 500 in association with a furtherfinite-state machine 260 of a Moore type that manages a loading signalL, which regulates storage of the 32-bit registers, i.e., the register230 of the counter 220 and the registers 242 _(i) of each i-th column240 _(i) of adders 241 _(i). The circuit 500 is shown only in partwithout the channel circuit 210. Consequently, for each k-th channelin_(k), signals ink_u and ink_d are represented that correspond to thecount signals cnt_up and cnt_down of FIG. 4. The loading signal L isshifted synchronously with the clock clk: in the example illustrated inFIG. 8, first, after an initial state IN, at the first clock tick clkcontrol of the FSM passes to a state L0, where the channel registers 230connected to the counters 220 are loaded with the output, i.e., thetotal number counts CT thereof. Then, at a next clock tick clk controlpasses to a next state L1 where the registers 242 ₁ of the first column240 ₁ are loaded with the sum of the respective adders 242 ₁, i.e., offour channels; then (state L2) the registers 240 ₂ of the second column240 ₂, which contain the sum of sixteen channels, are loaded; finally(state L3) the register 242 ₃ that contains the sum of the N=64 channelsis loaded. Control in the finite-state machine 260 then returns to theinitial state IN. In this way, it is possible to reduce the propagationtime necessary before an operation of storage in the registers 230, 242,which has to be made by circuit arrangement 500, can be carried out,instead of simply loading all the 85 registers 230, 242 _(i), of thecircuit at each clock pulse clk.

It should be noted that the latch operation between the output of thecounter and the input of the register, i.e., of storage, takes placesimultaneously for all the channels: the contents of the counters arecopied into the registers, once the count transitions are through.

Moreover, even though the acquisition must be carried out channel bychannel, the counters are stored in the registers all at the same time.Thus, once the cycle of the state machine 260 is through, the data inthe registers 230 and 242 refer to the same time.

Since the operation of acquisition is independent of the count of thepulses, the acquisition of the data does not stop the activity of thecounter 220, and hence there is no dead time on account of readout ofthe channels.

Each register 230 and 242 of the circuit arrangement 500 has available asingle bit of alarm signal that is activated when the most-significantbit of the counter (for a positive number and accordingly in two'scomplement for negative numbers), passes from 0 to 1. When an OR logicthat has as inputs all the signals corresponding to the variousregisters of the structure is equal to 1, an operation of asynchronousreset can be used for unloading the registers in order to preventoverflow and hence loss of data. In particular, the OR gate collects thealarm signal of all the registers 230 and 242 and indicates that one ofthe registers has exceeded half of its maximum capacity. It then enablesthe readout electronics for sending a reset signal before overflow isreached.

The circuit arrangement 500 described operates, for example, at a clockfrequency of 400 MHz and at a maximum conversion rate of 80 MHz, thecharge quantum Qc being selected in a range between 50 fC and 350 fC,thus enabling the limits of current of prior-art circuits to beovercome.

Hence, from what has been described, the solution and the correspondingadvantages emerge clearly.

The circuit arrangement described, thanks to the architecture that joinsbranches in parallel for each channel to an adder structure set betweenthe branches and the multiplexer, enables the limits of current of knowncircuits to be overcome.

The solution described herein makes it possible to obtain a uniformchannel-to-channel relative gain, i.e., the gain of one channel withrespect to the other, linearity over a wide dynamic range, andnegligible background currents, as well as the possibility of managing awider dynamic range of input currents, of the order of 10⁴. To obtainthese targets, a technology with large scale of integration is used,0.35-μm CMOS technology, in implementation of the ASIC, which enablesinstallation of the chips in the proximity of the detector.

The solution described herein can operate with inputs of bothpolarities, using 32-bit synchronous counters with up/down countcapacity. In this way, the charge can be measured by counting the pulsesat output from the channel converter in a given time interval. Theoutput of the channel circuit is hence already in digital format, whichenables a simpler subsequent management of the data.

The circuit arrangement described operates at a clock frequency of 400MHz and a maximum conversion rate of 80 MHz. The conversion of bipolarinputs makes it possible to extend the application of the circuitarrangement described to all the detectors that are used in thetechnical sector of calibration of beams of charged particles, andmoreover enables a better precision in the measurement of low-currentregimes (of the order of picoamps).

Of course, without prejudice to the principle of the invention, thedetails and the embodiments may vary, even considerably, with respect towhat has been described herein purely by way of example, without therebydeparting from the sphere of protection, which is defined by the annexedclaims.

The circuit arrangement described can operate with theionization-chamber sensors that envisage channels arranged in a stackthat develops along the direction of the beam, but can also operate withtwo-dimensional sensors, known as “pixel ionization-chamber detectors”,which comprise an array of cells or pixels in the plane orthogonal tothe beam, each of which corresponds to a channel and is hence designedto supply a current proportional to the dose that impinges on that cellor pixel, and in any case with all the ionization-chamber sensorssuitable for detecting the intensity of beams of charged particles andconfigured for sending their own measurement signal through a pluralityof measurement channels.

1. A circuit arrangement for acquisition of signals from an apparatusfor measuring beams of charged particles for external radiotherapy, inparticular protons, carbon ions, and other ion species, emitted byparticle accelerators, the circuit arrangement comprising at least oneionization-chamber sensor, said sensor comprising a plurality of sensorchannels, said circuit arrangement comprising a plurality of channelbranches in parallel designed to be connected to said sensor channelsfor receiving respective measurement signals (i) therefrom, said channelbranches comprising respective current-to-frequency converters andcounters for supplying count values, representing a charge associated toa given channel, to a multiplexer comprised in said circuit arrangement,said circuit arrangement wherein: it comprises an adder structureincluding at least one first column of adders that adds up the values atthe outputs of the channel branches for supplying sum values at outputsthereof; and said channel branches supply their own outputs directly tosaid multiplexer and to said adder structure, said multiplexer beingconfigured for selecting, from the direct outputs of the channelbranches and the outputs of the adder structure, a signal to be suppliedto a readout electronics, for reading of the channels, in particular inorder to enable the channel branches necessary for detecting a givenpeak excursion via the multilayer-ionization-chamber sensor withoutsaturating.
 2. The circuit arrangement according to claim 1, wherein itcomprises one or more further columns of adders that add up outputs ofthe adders of a previous column to form further outputs of the adderstructure connected to the inputs of said multiplexer.
 3. The circuitarrangement according to claim 1, wherein each channel branch comprisesa respective channel register connected for storing the value of countof the counter, and each adder is associated to a respective adderregister for storing the value of count of the adder.
 4. The circuitarrangement according to claim 3, wherein it comprises a finite-statemachine configured for storing the data in the channel registers and inthe registers of the adders of each column synchronously with a clocksignal (clk), at each tick of the clock signal storing data in sequencein the channel register or in each subsequent column of adders.
 5. Thecircuit arrangement according to claim 3, characterized in that eachchannel register and adder register has available an alarm signal thatis activated when the most-significant bit of the counter passes to highlevel, and a logic circuit, which receives at input each signal of eachchannel register and adder register configured for verifying whether allsaid alarm signals are at a high level and accordingly issuing acommand, in particular if so required, for an operation of asynchronousreset for unloading said registers in order to prevent a state ofoverflow.
 6. The circuit arrangement according to claim 1, wherein atleast one measurement signal (i) of one sensor channel is connected inparallel to at least one set of inputs and said multiplexer isconfigured for selecting the outputs of the adder structure that adds upthe values at the outputs of the channel branches that correspond tosaid set of inputs.
 7. An apparatus for measuring beams of chargedparticles for external radiotherapy comprising a circuit for readout ofthe channels comprising a circuit arrangement according to claim
 1. 8.The apparatus according to claim 7, wherein it comprises at least onesensor channel, the output of which is connected in parallel to at leastone set of inputs, and said multiplexer is configured for selecting theoutputs of the adder structure that adds up the values to the outputs ofthe channel branches that correspond to said set of inputs and forsupplying them to said channel-readout circuit.
 9. A method formeasuring beams of charged particles, in particular protons or ions,emitted by systems for external radiotherapy with charged particles thatuses a measurement apparatus according to claim
 6. 10. The methodaccording to claim 9, wherein it comprises: connecting a given channelof a sensor to a respective channel branch or to a set of channelbranches of said circuit arrangement that converges into one or morecolumns of adders as a function of the value of a maximum currentexpected on said channel of the sensor; configuring said multiplexer forselecting an output, corresponding to the connection of said givenchannel, from among the direct outputs of the channel branches and theoutputs of the adder structure, as signal to be supplied to a readoutelectronics for reading of the channels.
 11. The method according toclaim 10, wherein said operation of connecting a given channel of asensor to a respective branch or to a set of branches of said circuitarrangement comprises selecting, for the connection, a set of brancheshaving a number of branches such as to divide on each branch a currentlower than a current of saturation of the channel circuit.